This invention relates to detectors for measuring the energy of elementary particles, specifically to a superconducting transition-edge sensor with enhanced electrothermal feedback.
One example of an elementary particle is the Photon, which is the individual quantum of electromagnetic radiation. Known forms of electromagnetic radiation, arranged in the order of increasing energy and decreasing wavelength, include radio waves, microwaves, infrared (IR) radiation, visible light, ultraviolet (UV) radiation, X-rays, and gamma rays. High-resolution elementary-particle detectors have many applications, such as infrared bolometry, X-ray spectroscopy in the area of material science, X-ray astronomy, and optical-photon detection. Important goals in the design of such detectors include higher maximum count rates as well as an improvement in resolution by linearizing the detector through negative electrothermal feedback.
Some varieties of elementary-particle detectors are based on the principle of superconductivity, which is a low-temperature phenomenon where the resistance of certain materials drops essentially to zero at a critical transition temperature typically near absolute zero. One type of known superconducting sensor capable of detecting elementary particles, including individual photons from the infrared (IR) through the gamma regions of the electromagnetic spectrum, is the transition-edge sensor (TES). The transition-edge sensor operates most efficiently when its detector element serves as an active component in an electrothermal-feedback (ETF) loop. The detector element of a TES comprises a thin film made of a superconducting material and is designed to operate within a temperature range .DELTA.T (FIG. 1), which defines the superconducting transition in electrical conductance for that material. Critical temperature T.sub.c marks the middle of the temperature range .DELTA.T. In the superconducting-transition temperature range .DELTA.T, the detector element has the properties of a temperature-sensitive resistor with a high positive temperature coefficient of electrical resistance. The temperature coefficient of electrical resistance represents the amount of change that occurs in the resistance R of the detector element as a result of a change in temperature T. A related quantity, .alpha..sub.o, defined as either (T/R)(dR/dT) or d InR/d InT, is a unitless measure of the sharpness of the superconducting transition of the detector element.
Generally, a heat sink is placed in thermal contact with the detector element for dissipating the energy of the detector. The temperature of the detector element is maintained within its superconducting-transition region by applying an appropriate voltage bias across the detector element. The temperature of the heat sink is held well below T.sub.c. The voltage bias produces a current through the detector, resulting in a heating effect, known as the Joule effect or Joule heating. Thermal equilibrium of the detector element is achieved by matching the Joule heating of the detector element with the heat dissipated from the detector to the heat sink, which, to first order, remains constant. As apparent from FIG. 1, due to a high positive temperature coefficient of electrical resistance the detector element possesses in its superconducting-transition region, a small change in the temperature of the detector element responsive to energy deposited into the detector element, e.g., by radiation directly incident upon the detector, is accompanied by a large change in electrical resistance. The increase in the resistance of the detector results in a decrease of the current flow therethrough and hence a decrease in Joule heating. The temperature of the detector element therefore decreases and its thermal equilibrium is re-established with the help of a negative electrothermal-feedback loop where the current through the detector element and the corresponding Joule heating thereof are inversely proportional to the detector's electrical resistance (Joule heating=V.sup.2 /R where V is the voltage bias across the detector and R is the resistance of the detector element). Alternatively stated, the voltage bias across the detector element creates a current therethrough that is sufficient to raise the temperature of the detector element by Joule heating to a level within the superconducting transition where a further increase in the temperature of the detector element, corresponding to energy deposited into the detector element by, e.g., an incident radiation particle, reduces the Joule heating by increasing the electrical resistance of the detector element and hence reduces the current through the detector element, thus stabilizing the temperature thereof. Thus, it is primarily the reduction in Joule heating which compensates for the energy increase of the detector element due to a particle incident thereon. The observed result is a current pulse with a pulse-decay time considerably shorter than the intrinsic pulse-decay time determined by the heat capacity of the detector element and the thermal conductivity of the heat sink. Since the energy of a particle incident on the detector element can be calculated by integrating the changes in the current though the detector element occurring over time, variations in the current through the detector element are measured to provide a signal indicative of the energy of the particle.
The higher the positive temperature coefficient of the material comprising the detector element of the TES, the faster the temperature of that detector element will stabilize following an energy pulse deposited into the detector element by a particle incident thereon, thus allowing a TES to have shorter pulse-recovery times and higher count rates. However, the temperature coefficient of electrical resistance is also the limiting factor with respect to the pulse-recovery time of the sensor because this temperature coefficient is a fixed quantity that is a function of the detector material. Thus, the count rates of which the sensor is capable approach a theoretical limit determined by the temperature coefficient of the detector material in its superconducting-transition region.